OA21203A

OA21203A - Drill string joint design.

Info

Publication number: OA21203A
Application number: OA1202300100
Authority: OA
Inventors: Tomas Jansson; Anders Nordberg
Original assignee: Sandvik Mining And Construction Tools Ab
Priority date: 2020-09-17
Filing date: 2021-09-16
Publication date: 2024-02-29

Abstract

A drill string rod (1) to form part of an assembly (10) of connected such drill string rods (1, 1b), the drill string rod (1) comprising: an elongate central rod portion (2) extending axially between a male end (3) and a female end (4), wherein the central rod portion (2) is hollow-cylindrical defined by an inner first diameter (dr0d) and an outer second diameter (Dr0d), wherein the male end (3) comprises a spigot (5),wherein the spigot (5) comprises a base (6) projecting axially from a shoulder (7) that axially separates the spigot (5) and the central rod portion (2), wherein the female end (4) comprises a sleeve portion (8) configured to fit to the spigot (5), wherein the base (6) is provided with an outer thread and wherein the sleeve portion (8) is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion (8) is attachable to the outer thread of the base (6) of the spigot (5) of a further drill string rod (1 b) of the assembly (10), wherein, in a radial plane (P) to the longitudinal axis (9) of the drill string rod (1), the base (6) of the spigot (5) is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion (8) is defined by an outer fifth diameter (Dsleeve) and an 20 inner sixth diameter (dsleeve), wherein the second diameter (Drod) is between 30 and 60 mm, and wherein the six diameters mentioned are selected within specific constraints.

Description

Title: Drill string joint design

Technical field

The présent invention relates to drill string rods for use with drill bits for percussion rock drilling. Specifically, the invention relates to improving the reliability and longevity of such drill string rods.

Background

Percussion drilling is used to create a long borehole via a plurality of elongate drill string rods coupled together end-to-end by interconnected male and female threaded ends. The well-established technique breaks rock by hammering impacts transferred from a rock drill bit, mounted at one end of the drill string, to the rock at the bottom of the borehole. Typically, the energy required to break the rock is generated by a hydraulically driven piston that contacts the end of the drill string (via a shank adaptor) to create a stress (or shock) wave that propagates through the drill string and ultimately to the rock. Conventional maie and female threaded couplings are described in US 4,332,502; US 4,398,756; US 4,687,368 and DE 2800887.

The male and female threaded ends of neighboring drill rods are coupled to create the drill string and the joint is typically subjected to large forces during drilling. These forces fatigue the coupling and lead to wear and breakage within the threaded portion of the joint. Typically, it is the threaded male spigot that is damaged and détermines the operational iifetime of the coupling. US 6,767,156 discloses a threaded joint between two percussive drill rods having conical guiding surfaces provided at the leading axial ends of the male and female portions in an attempt to achieve a secure coupling and prevent damage to the threads.

EP2845992B1 by the présent applicant aims to mitigate the abovementioned drawbacks.

One problem with known drill string joint designs is that failure occurs due to bending which originates from non-straight holes. Cracking may occur is the spigot or in the sleeve portion therefore reducing the reliability and longevity of the drill string rods.

Hence, there is a need for a drill string joint design having reduced 5 stress and increased bending résistance, therefore improving the reliability and increasing the lifetime of the drill string rods.

Summary

An object of the invention is to provide a drill string rod with improved reliability and longevity of the drill string rods. According to a first aspect of the invention, this object is achieved by the inventive drill string rod as defmed in the appended independent claim 1, with alternative embodiments defined in the dépendent daims. The drill string rod is to form part of an assembly of connected such drill string rods. The drill string rod comprises an elongate central rod portion extending axially between a male end and a female end. The central rod portion is hollow-cylindrical defined by an inner first diameter (d_rod) and an outer second diameter (Drod). The male end comprises a spigot, wherein the spigot comprises a base projecting axially from a shoulder that axially séparâtes the spigot and the central rod portion. The female end comprises a sleeve portion configured to fit to the spigot. Also, the base is provided with an outer thread and the sleeve portion is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion is attachable to the outer thread of the base of the spigot of a further drill string rod of the assembly. In a radial plane to the longitudinal axis of the drill string rod, the base of the spigot is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion is defined by an outer fifth diameter (Dsieeve) and an inner sixth diameter (dsieeve). The présent invention is limited to drill string rods with an outer second diameter (D_rOd) between 30 and 60 mm, preferably between 40 and 60 mm. Csieeve and Cspigot are related to the diameters of the drill string rod as defined by the following formulas:

„ _Droit ‘ (Psleeve ^-sleevù ^SleeVe D_sleeve · W_od Drod ' (Pspigot ^— ^-spigot) ^Lspigot — _n . rn4 _ λ4 \ ^uspigot k^urod ^urodJ

Csieeve should be larger than 2.0 or C_spigotShould be larger than > 0.6.

The skilled person understands that the diameters cannot be chosen freely. For example, the outer diameter of the spigot is naturally limited by the inner diameter of the sleeve portion. Also, the outer diameter of the sleeve portion is typically limited by the size of the bore drilled and the requirement for enough space for the flushing away of drill cuttings past the sleeve portion between the sleeve portion and the inner wall of the bore. Similarly, the inner diameter of the spigot limits the rate at which flushing fluid can be pumped through the drill string rod, and thus should not be too small. The design of drill string rods is clearly a difficult balancing act and the présent invention guides the skilled person to the sélection of a combination of parameters enabling reduced risk of material failures in the drill string rod with sustained flushing performance. To solve this problem has shown to be more difficult than it would seem.

The inventors of the présent invention hâve realized that the drill string rod failures are due to the combination of the increased stiffness of the central rod portion and forced bending of the drill string induced by harder layers or cracks in the rock. The bending of the drill string rod happens while the rod is rotating and thereby créâtes both additional stress due to bending, and fatigue due to the constant change of bending axis caused by the rotation of the rod in the bore whilst in its bent condition.

The proposée! solution is to dimension the spigot and the sleeve portion such that the calculated maximum bending stress in the spigot and the sleeve portion respectively is related to the calculated maximum bending stress in the central rod portion. However, the inventors hâve realized that the calculated maximum stresses of the sleeve portion and the spigot respectively should not be equal to the calculated maximum bending stress of the central rod portion but rather relate to the calculated maximum bending stress of the central rod portion multiplied by the factor Csieeve or Cspigot, respectively, in order to account for commonly occurring différences in strength occurring for example due to local material variations likely occurring due to uneven hardening and/or machining at manufacturing of the drill string rods.

In some embodiments, Csieeve > 2.0 and Cspigot > 0.6. This balance of the first, second, third, fourth, fifth, and sixth diameters invoived provide for high reliability and longevity of the drill string rod.

In some embodiments, the base of the spigot is conical.

In some embodiments, the base of the spigot is cylindrical.

In some embodiments, the first diameter (d_rod) is 30 mm, wherein the second diameter (D_rOd) is 57 mm, wherein the third diameter is (D_spigot) is 48 mm, wherein the fourth diameter (d_spigot) is 21 mm, wherein the firth diameter is (D_si_eeve) is 76 mm and wherein the sixth diameter (d_Si_eeVe) is 51 mm.

In some embodiments, the drill string rod is suitable for use with a drill bit having an outer seventh diameter (Dhoie), wherein the fifth diameter is < 0,90* the seventh diameter (Dhoie).

A further aspect relates to a System comprising a plurality of drill string rods according to the first aspect described above.

In some embodiments, the drill string rods of the System are of the type mentioned above which are suitable for use with a drill bit having an outer seventh diameter (Dhoie), wherein the fifth diameter is < 0,90* the seventh diameter (Dhoie), and wherein the System further comprises the drill bit.

The différence in diameter between the hole and the outer diameter of the sleeve portion enabies efficient flushing of drill cuttings 5 past the sleeve portion, whilst providing robust drill string rods capable of handling the forces involved.

In some embodiments, the seventh diameter is > 89 mm. The drill string rods hâve a second diameter between 30 and 60 mm, preferably between 40 and 60 mm, wherein the specified constraints of the involved diameters of the drill string rods governed by Csieeve and Cspigot provide for a robust drilling System.

In some embodiments, the threads on the base and the sleeve portion are both straight.

In some embodiments, the threads on the base and the sleeve 15 portion are both cambered.

Description of drawings

Fig. 1 shows a perspective view of two identical connected drill string rods.

Fig. 2 shows an enlarged perspective view A of the male end of the drill string rod as indicated in Fig. 1.

Fig. 3 shows an enlarged perspective view B of the female end of the drill string rod as indicated in Fig. 1, which is connected to the male end of the other drill string rod.

Fig. 4 shows a cross-sectional side view of the sleeve portion and the spigot of the two connected drill string rods also shown in Fig. 1, said cross-section taken in a plane through the longitudinal central axis of the drill string rod.

1	drill string rod	9	longitudinal axis of drill string rod
1 b	further drill string rod of same type	10	system/assembly of a plurality of drill string rods
2	central rod portion	P	plane defining cross sections for determining C_Spi_got and C_si_eeve
3	male end	drod	inner diameter of rod / first diameter
4	female end	Drod	outer diameter of rod / second diameter
5	spigot	Dspigot	outer diameter or spigot / third diameter
6	base (of spigot)	dspigot	inner diameter of spigot / fourth diameter
7	shoulder	Dsleev e	outer diameter of sleeve portion / fifth diameter
8	sleeve portion	d sleeve	inner diameter of sleeve portion / sixth diameter
		Dhole	Diameter of hole drilled /seventh diameter

Detailed description

A first embodiment of the invention is shown in Figs. 1 -4. As shown in the figures, a plurality of identical drill string rods 1,1 b are connectable to form a system/assembly 10. The drill string rod 1 comprises an elongate central rod portion 2 extending axially between a male end 3 and a female end 4. As shown in Fig. 4, the central rod portion 2 is hollow-cylindrical defined by an inner first diameter drod and an outer second diameter D_rod. The male end 3 comprises a spigot 5, and the spigot comprises a base 6 projecting axially from a shoulder 7 that axially séparâtes the spigot 5 and the central rod portion 2. The female end 4 comprises a sleeve/sleeve portion 8 configured to fit to the spigot 5. The base 6 is provided with an outer thread and the sleeve portion 8 is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion 8 is attachable to the outer thread of the base 6 of the spigot 5 of a further drill string rod of the assembly. In a radial plane P to the longitudinal axis of the drill string rod the base 6 of the spigot 5 is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion 8 is defined by an outer fifth diameter (Dsieeve) and an inner sixth diameter (dsieeve). It should be noted that in Fig. 4, Dspigot and dsieeve are both mentioned in connection with the same arrow although in reality the diameters are slightly different with dsieeve being larger than Dspigot. The scale of the drawing is such that the différence in diameter cannot be shown in Fig. 4 using separate arrows. Although the diameters may vary along the length of the spigot and the sleeve portion respectively, the radial plane

P which by necessity runs through both the spigot and the sleeve portion, is used to define the relationship between the diameters in an unambiguous way. The présent invention is limited to drill string rods with an outer diameter between 30 and 60 mm, preferably between 40 and 60 mm such as drill bits having a diameter of 54 -127 mm. Hence, the second diameter (D_rod) is between 30 - 60 mm, preferably between 40 - 60 mm . Csieeve and Cspigot are derivable from/related to the diameters of the drill string rod using the following formulas:

These formulas stem from the calculation of section modulus for 30 hollow-cylindrical beams/bodies which provide a good approximation of the présent involved cross-sectional shapes. Specifically, the section modulus équations for hollow-cylindrical cross-sections is:

_ D_rod ' (fisleeve d_sieeve) ^sleeve — _n . r_n4 _ j4 λ ^usleeve V^rod ^LlrodJ

Drod ' (Pspigot ^— ^-spigot)

Tr^-dj) 4r2 32^

Fig 5 shows a section modulus hollow round center neutral.

Our assumption is that:

C _ C *Ç ^ⁿ(^Dspigot~^dspigot) _ ^(Drod~^arod) * _r x ùspigot - Upigot brod^ 32*D_spigot· “ 32*Drod ^Lspigot x λ’ _ Drod'(Dspigot~^dspigot) ^spigot ~ _Dspigot.^_od^_rod)

And similarly that Ssleeve⁼Cs!eeve*Srod „ _ Drod ‘ (Psleeve ^-sleeve^

-> ^sleeve — _n . _fn4 _ j4 \ ^sleeve V^rod ^urod2

In this embodiment, the first diameter d_rad is 30 mm, wherein the second diameter D_rOd is 57 mm, wherein the third diameter D_spigot is 48 mm, wherein the fourth diameter d_spigot is 21 mm, wherein the firth diameter D_si_eeve is 76 mm and wherein the sixth diameter d_si_eeve is 51 mm.

This embodiment of the drill string rod 1,1 b is suitable for use with a drill bit (not illustrated) having a specified diameter (the diameter not including the drilling inserts) of 54, 57, 64 70, 76, 89,102,115 or 127 mm. The actual diameter of the holes drilled is slightly larger since the drill bits protrude radially. The first through sixth diameters may in other .

embodiments be chosen differently, as long as the second diameter D_rod, which defines the outer diameter of the central rod portion 2, fulfills the above constraint of being between 30 and 60 mm, and the other diameters fulfil the constraints that Csieeve is larger than 2.0 or that Cspigot is larger than 5 0.6. Preferably, Csieeve is larger than 2.0 and Cspigot is larger than 0.6.

The drill string rods are made of a suitable material, such as steel, and are hardened as needed.

The drill string rod 1,1 b is dimensioned based on the size of the drill bit for which it is to be used. However, care must be taken to provide 10 enough space around the sleeve portion 8 for drill cuttings to be flushed past the sleeve portion 8. To this effect, the drill string rod 1,1b may in some embodiments be suitable for use with a drill bit having an outer seventh diameter Dhoie, wherein the outer fifth diameter Dsieeve of the sleeve portion 8 is less than 0,90* the seventh diameter Dhoie.

A plurality of identical drill string rods 1,1b may be provided together as part of a system/assembly 10 of the drill string rods 1,1b. The System 10 may altematively comprise a drill bit. The drill string rods 1,1b of the System 10 may hâve an outer seventh diameter Dhoie, wherein the fifth diameter is < 0,90* the seventh diameter Dhoie, and wherein the

System further comprises the drill .

bit.

The différence in diameter between the hole and the outer fifth diameter Dsieeve of the sleeve portion 8 enables efficient flushing of drill cuttings past the sleeve portion 8, whilst providing robust drill string rods 1, 25 1b capable of handling the forces involved.

In some embodiments, the seventh diameter Dhoie may be 54 -127 mm. As mentioned above, the drill string rods of the présent invention ail hâve a second diameter between 30 and 60 mm, preferably between 40 and 60 mmThe specified constraints of the involved diameters given by 30 Csieeve and Cspigot provide for a robust system/assembly 10.

Claims

1. A drill string rod (1) to form part of an assembly (10) of connected such drill string rods (1,1b), the drill string rod (1) comprising:

an elongate central rod portion (2) extending axially between a male end (3) and a female end (4), wherein the central rod portion (2) is hollow-cylindrical defined by an inner first diameter (drod) and an outer second diameter (Drod), wherein the male end (3) comprises a spigot (5), wherein the spigot (5) comprises a base (6) projecting axially from a shoulder (7) that axially séparâtes the spigot (5) and the central rod portion (2), wherein the female end (4) comprises a sleeve portion (8) configured tofitto the spigot (5), wherein the base (6) is provided with an outer thread and wherein the sleeve portion (8) is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion (8) is attachable to the outer thread of the base (6) of the spigot (5) of a further drill string rod (1 b) of the assembly (10), wherein, in a radial plane (P) to the longitudinal axis (9) of the drill string rod (1), the base (6) of the spigot (5) is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion (8) is defined by an outer fifth diameter (Dsieeve) and an inner sixth diameter (dsieeve), wherein the second diameter (Drod) between 30-60 mm, wherein:

« _Drod ’ (Dsieeve ^-sleeve) ' ^sleeve ~ . <-_n4 _ j4 a ^sleeve \^urod ^urodJ „ _ D_rod ' {Pspigot ^— ^spigot) ^spigot - _n · rn⁴ - rf⁴ Ί ^spigot v^rod ^urodJ wherein, and wherein Csieeve > 2.0 or Cspigot > 0.6.

2. A drill string rod (1) according to claim 1, wherein Csieeve > 2.0 and Cspigot > 0.6.

3. A drill string rod (1) according to any one of daims 1 -2, wherein the base 5 (6) of the spigot (5) is conical.

4. A drill string rod (2) according to any one of daims 1 -2, wherein the base (6) of the spigot (5) is cylindrical.

10

5. A drill string rod (1) according to claim 1 or 4, wherein the first diameter (drod) is 30 mm, wherein the second diameter (Drod) is 57 mm, wherein the third diameter (D_Spi_got) is 48 mm, wherein the fourth diameter (d_spigot) is 21 mm, wherein the fifth diameter (Dsieeve) is 76 mm and wherein the sixth diameter (dsieeve) is 51 mm.

6. A drill string rod (1) according to any one of daims 1-6, wherein the drill string rod (1) is suitable for use with a drill bit having an outer seventh diameter (Dhoie), wherein the fifth diameter (Dsieeve) is < 0,90* the seventh diameter (Dhoie).

7. System (10) comprising a plurality of drill string rods (1,1b) according to any one of the preceding daims.

8. System (10) according to claim 7 when dépendent on claim 6, wherein said 25 System (10) comprises the drill bit.

9. System (10) according to claim 8, wherein the seventh diameter (Dhoie) is 54-127 mm or larger.

30

10. System (10) according to any of the previous daims, wherein the second diameter (Drod) between 40-60 mm.

11. System (10) according to any of the previous claims, wherein the threads on the base (6) and the sleeve portion (8) are both straight.

12. System (10) according to any of claims 1-10, wherein the threads on the

5 base (6) and the sleeve portion (8) are both cambered.